Simulation has given scientists a glimpse at how some unusual crystalline solids swell to more than triple their normal volume in the presence of organic solvents (Science2007, 315, 1828). Although crystalline solids are normally rigid, a small group of organic-inorganic versions swell dramatically and reversibly in response to gas or solvent adsorption. This feature makes them attractive for applications in selective adsorption and storage. Structural studies have revealed what these flexible solids look like in the deflated and inflated states, but they have not been able to shed light on the molecular process by which guest molecules trigger swelling. So Christian Serre, Gérard Férey, and coworkers at the University of Versailles, in France, used simulations and crystal structure data to provide an atomic-scale description of the structural changes involved in the unusually large swelling observed in a series of Cr(III) and Fe(III) dicarboxylate solids upon uptake of organic solvents. No bonds are broken as these metal-organic frameworks take up solvent. Instead, the researchers find that rotations of the dicarboxylate linkers allow the structure to breathe and incorporate solvent molecules.